Clark McClelland claims to have observed the eight to nine-feet tall being in an astronaut’s suit in the bay of the shuttle.

The story, which has emerged on paranormal websites and YouTube, is true he said, giving a testimony about the supposed sighting in a vide for YouTube channel Paranormal Crucible.

The channel produced mock-up video of the oversized “alien entity” interacting with NASA astronauts in the bay.

Mr McClelland claims he was monitoring the shuttle from the Kennedy Space Centre in Cape Canaveral, Florida, at the time of the sighting in 1991.

Mr McClelland described while monitoring an unspecified mission from the space centre’s launch control centre on a 27-inch video screen, he saw the “alien” standing upright on two legs in the space shuttle payload bay, interacting with the two tethered US astronauts.

He said he watched it for one minute and seven seconds.

He says on another occasion he also saw a UFO at the rear of the shuttle.

He has detailed the experiences on his website www.stargate-chronicles.com, where he also claims to have been involved in 800-plus NASA operations, including the Apollo missions, the launching of the International Space Station and the space shuttle programme.

The website includes a picture labelled as him with Judy Resnick, the second ever US female astronaut who died in the 1986 Challenger space shuttle disaster alongside six others.

Some other UFO hunters describe his story as being some of the “most conclusive” evidence for allegations that aliens exist and have visited Earth, but that there is a global government cover-up to keep the truth from us.

In one video he said: “I know an ET and alien craft when I see them. Aliens are here on Earth, they walk among us. They may have been implanted into our various Earth governments.”

Mr McClelland’s supporters say the US space agency has never refuted his story.

But a sceptic posted on a forum on the Abovetopsecret.com site which looks at “alternative topics” and conspiracy theories, questioning the validity of his claims.

He asked: “Why is it if you google his name it only comes up on UFO websites, or postings in forums where the topic of the post was UFOs? There are no mentions of him on NASA’s site.”